This invention relates generally to radio frequency receivers and more particularly to wide bandwidth radio frequency receivers.
As is known in the art, wide bandwidth radio frequency receivers have many applications, as, for example, in radio frequency signal direction finding apparatus. One such receiver suggested for such application is a homodyne receiver wherein a first portion of a received signal is fed to a mixer together with a second portion of the received signal. Generally, one of the portions of the received signal is translated in frequency to a fixed intermediate frequency (IF) with the result that any signal received by the receiver over an extremely wide band of frequencies appears at the output of the mixer at the fixed IF frequency. This output signal is then processed by subsequent signal processing apparatus designed to operate in response to signals at the fixed IF signal. Consequently, such processing apparatus is adapted to process received signals having a frequency within an extremely wide bandwidth. In a practical environment, however, the received signal is in the presence of background noise. Since the noise typically has a wide bandwidth, the frequency spectrum of the noise is spread over the entire receiver bandwidth. It follows, then, that the larger the receiver bandwidth, the larger the amount of noise received by the receiver, thereby reducing the sensitivity of the receiver. In particular, in the homodyne receiver described above, since noise is fed to the mixer along with the first and second portions of received signal, the amount of noise produced at the output of the mixer at the intermediate frequency is proportional to the square root of the bandwidth of the receiver. Consequently, receiver bandwidth must be compromised by noise, and hence receiver sensitivity, considerations.
A second type of wide bandwidth receiver system is a so-called multi-channel receiver. Here a plurality of narrow bandwidth receivers each heterodynes the received signal to a corresponding plurality of intermediate frequencies which pass through the receivers. While such multi-channel receiver systems have a sensitivity related to the bandwidth of any one of the relatively narrow bandwidth receivers, such systems are relatively expensive since they require a large number of individual receivers.